Slurry Oil/Catalyst Fines Tank Bottoms Recovery & Processing
The problems presented by catalyst attrition from fluid catalytic cracking units (FCCU) have plagued the refining industry since the advent of fluid catalytic cracking in the first half of the 20th century. Over time, FCCU catalyst deteriorates in size. The size deteriorated catalyst is commonly referred to as catalyst fines.
In the FCCU process, cracked product stream vapor and some catalyst (typically of less than 20 microns in diameter) leave the reactor and enter the main fractionator near its base. On most units the bottom stream from the fractionator is called heavy cycle oil (HCO) or slurry oil. HCO has a typical gravity of from about −4.0 API to about 3 API. For purposes of the present invention, it is sufficient to know that catalyst fines of about 20 microns in diameter or less, make their way to the slurry oil product storage tank. Slurry oil is a saleable product of FCCU processing. Once in the storage tank, the catalyst fines settle to the bottom, albeit very slowly.
The existence of catalyst fines in the slurry tank presents a variety of problems to the refiner. The immediate and obvious problem has to do with product contamination. Slurry oil has proven to be an ideal feedstock for carbon black manufacture. Utilization as carbon black feedstock maximizes the value of slurry oil product. However, the presence of catalyst fines above a specified percentage in the slurry oil product results in an “ash content” specification in excess of that which is acceptable for use of the slurry oil as a carbon black feedstock. Even when the slurry oil product is utilized as a fuel source, a “price penalty” is, effectively, rendered by the market place as a result of ash content, in the form of the inorganic catalyst fines.
Firms within the specialty chemical industry, which service the petroleum refining industry, have built proprietary product lines that serve to enhance settling of the catalyst fines. In recently or relatively recently cleaned storage tanks this procedure is typically successful in enabling the stored slurry oil product to meet even the rigorous specifications of carbon black manufacturers. As the accumulation of catalyst fines continues in the storage tank, a time comes when no amount of settling enhancement will permit the stored product to “meet specifications” of carbon black manufacturers or even fuel products.
When accumulation of catalyst fines in the slurry oil storage tank becomes intolerable, in terms of meeting product specification, refinery management schedules a clean-out. The clean-out is conducted under one of two typical scenarios. One type of clean-out calls for the removal of the catalyst fines without human entry. In this instance, enough of the catalyst fine sediment is removed to make the bottoms manageable once again. The second type of clean-out entails a complete removal of all catalyst fine sediment, subsequent human entry for complete clean up, a so-called mop-up, all followed by inspection, repairs and return-to-service.
The low API/high density of the slurry oil, coupled with the entrained catalyst fines, contributes to recovery and handling problems that are reputed to be some of the toughest in the tank cleaning industry. The tank cleaning industry has devised a number of procedures for catalyst fine removal from slurry oil storage tanks. These include the injection of diluent at high pressure either via side ports or from the roof, the cutting of “door sheets” using a water torch and various probe insertion devices. One such insertion device was co-invented by the present inventor and is called the SWEEPBER and is the subject of U.S. Pat. No. 6,142,160, which serves the purpose of recovering catalyst fines from the bottom of slurry oil storage vessels. A diluent is required to enhance ease of handling of the catalyst fine bottoms in all instances known to this inventor. The observed diluent of choice is Light Cycle Oil or LCO, a side-cut of the FCCU fractionator.
The overwhelming preponderance of catalyst fine projects are then conducted in a manner described as follows: As removal from the tank is carried out the typical procedure calls for transfer of the slurry oil/catalyst fines/diluent mixture (hereinafter “SCDM”) to a mobile mix tank, such as that supplied by Baker Tanks Inc., of approximately 22,000 gallons (approximately 500 barrels) capacity. The mix tank has the capability of heating the contents. A heated catalyst fine suspension of pre-specified temperature and concentration is then prepared, in the mix tank, as feed for centrifuge processing.
The heated feed is charged to the centrifuge and processed at a typical rate of 35 gallons per minute to 42 gallons per minute. Two streams result from the centrifuge process. One stream is referred to as recovered oil; the second stream is referred to as “filter cake”. The recovered oil is utilized per refinery management discretion. A typical option is to blend the recovered oil into heavy fuel oil products.
Pursuant to current U.S. Environmental Protection Agency guidelines, the filter cake is considered a hazardous waste. The cost of hazardous waste disposal has risen by ten fold in the last decade and is expected to continue rising. The principal specification that governs the acceptability of filter cake for disposal to a hazardous waste landfill is the “paint filter test”. This test requires the absence of free flowing oil through a standard filter. However, despite the absence of free-flowing oil within the filter cake, a substantial amount of hydrocarbon content remains within the filter cake and, thus, goes unutilized.
It is not unusual to find that the true hydrocarbon content of post-centrifuged filter cake is greater than 50%. It has been observed that filter cake of high melting point hydrocarbons, such as slurry oil, may contain as much as 83% hydrocarbon. The determination of true hydrocarbon content may be found by conducting a standard ASTM procedure for oil and grease or a true distillation.
There is a currently-used, second method of disposal for filter cake that renders the cake non-hazardous under EPA guidelines. The method is described in U.S. Pat. No. 5,443,717 to Robert M. Scalliet, et al. entitled “Recycle of Waste Streams”.
The Refinery Desalting Process
On the front line of defense in preventing refinery, process-side corrosion and processing unit contamination is the crude unit desalter. Despite the name, the desalter serves two principal functions: A) to minimize the chloride contamination and contamination by other water soluble, deleterious chemistries, found in raw crude oil, by precluding their introduction into the crude unit and downstream processes and B) to minimize and/or preclude the introduction of so-called “Basic Sediment and Water” (BS&W) into the crude unit and downstream processes.
The desalting process takes place in the desalter vessel. The desalter may be likened to a crude oil washing machine. Simply described, the desalting process consists of adding wash water to raw crude oil and then mixing the wash water with the raw crude such that the water makes contact with both soluble chemical contaminants and insoluble sediments. The wash water extracts the inorganic salts and other water-soluble chemistries. Further, under ideal conditions, the wash water serves to “water wet” insoluble sediments rendering them hydrophilic.
The objective to desalting optimization is to bring about a resolution of the oil water emulsion that has been purposely created by injecting a water wash into the crude charge prior to the mix valve. Two of the principal contributors to dehydration of the emulsified raw crude oil, not necessarily in order of importance, are A) the application of a treatment additive, commonly referred to as a demulsifier, that serves to promote coalescence of the water and B) the passing of the crude oil emulsion through an electric field, created within the desalter, that serves to enhance an electrostatic coalescing process.
The two factors previously referenced, the treatment additive and the electrostatic coalescing field, are by no means the only contributors to desalter opt optimization. Additional parameters that contribute to desalter optimization are referenced in the section herein below entitled: Translating Bench Model Results To Commercial Scale Practice. When all variables are set satisfactorily, dehydration of the crude emulsion will occur with a simultaneous migration of the cat' fine component of the SCDM into the water phase of the desalting process. The mechanism for dehydration is suggested by a coalescence of water droplets, which settle according to Stokes Law. As settling of the water occurs, both soluble contaminants and water-wetted sediments are carried downward, out of the hydrocarbon phase and into the lower water layer, which is maintained in the desalter. In the desalter, this process takes place on a continuous basis with dehydrated hydrocarbon rising upward and out of the top of the desalter vessel while, simultaneously, water settles downward and is pumped out of the desalter and through piping at the desalter bottom. The so-called desalter effluent water carries with it both the soluble chemical contaminants and the water-wetted, insoluble sediment in the form of the original crude oil inorganic contaminants and cat' fines introduced by the SCDM.
Prior to being charged into the desalter, a water wash is injected into the crude stream. The wash water is mixed into the crude by means of a special piece of hardware termed a mix valve. The mix valve is designed to create a repeatable mixing shear such that the wash water and raw crude oil may be mixed in a predictable manner that can be duplicated and repeated. The determination of the precise mix valve setting is paramount to the achievement of desalter optimization as is the amount and source of the water wash. The previous description of the desalting process, which is essentially a deliberate emulsification followed by a dehydration process, is widely held to be as much an art as a science by those professionals who specialize in the craft of optimizing desalter operation.